Many operations performed by machines require a high degree of positional accuracy between different elements of the machine to ensure accurate manipulation or handling of machine components or workpieces. For instance, machine tool structures typically require great rigidity to maintain an accurate relative position between the cutting tool and the workpiece. This has traditionally been accomplished by using massive castings or weldments having the requisite rigidity.
The weight or mass of these machine components is problematic because it limits operational and design capabilities. For example, the massive castings or weldments cannot be moved at high velocity or accelerated at a rate sufficient to meet certain modern machining requirements. Additionally, many modern machining operations require complex movements of the tool, and the corresponding manipulation of massive structures can tremendously inhibit the function and precision of the machine. However, if lighter structures are used, the stiffness of the machine is sacrificed and the machining operations are less accurate. Insufficiently rigid structures cause instability in machine operation that can lead to undesirable affects such as tool chatter.
Also, as tolerance requirements for machined components continually rise, this instability and lack of accuracy become even more unacceptable. Along with requirements for greater accuracy, there is continual demand for higher production rates requiring greater tool speed and greater tool acceleration. For example, rapid acceleration is required when the cutting tool is moved quickly around a small radius. It is difficult to accurately accelerate existing machine tool structures in this way due to either the mass of the casting or weldment or the lack of structural rigidity in less massive conventional structures.
Currently, some machine tools have incorporated stationary frameworks which reduce the overall mass while maintaining sufficient rigidity for at least certain machine operations. For example, at least one design incorporates a carrying structure which is a closed monocoque body type. This design has an outer stationary framework comprising vertical supports connected to cross pieces. The volume inside this structure is the machine work zone and is sufficiently sized to permit movement of a tool head and sliding table therethrough. However, this design remains limited by the relatively large and stationary framework which must cooperate with sufficiently rigid machine tool heads and workpiece platforms to avoid the flexing which leads to inaccurate machining. The stationary framework is designed to decrease shipping weight rather than to provide a lightweight framework designed to move with a machine component.
In another design, a stationary framework for a machine tool is provided in the form of a tetrahedral structure. This design, again, uses a large stationary frame which reduces shipping weight, but it cannot be moved with an attached machine component. The actual tool head and workpiece holder are disposed within this large tetrahedral frame and are manipulated while the framework remains stationary. Another problem with this type of design is that the framework is subject to flexural stress due to the transfer of forces from the tool head to the workpiece supports. This is unacceptable for precise machining operations.
In another design, an external stationary octahedral machine tool frame is used to provide rigidity to the overall framework of a machine tool while lessening the actual mass. In this design, an external, octahedral frame supports both a work table and servostruts which, in turn, support the spindle head. This design fails to disclose a lightweight, rigid, moveable truss-based framework capable of maintaining positional constancy between elements in a machine. The external, octahedral frame decreases shipping weight but it does not improve performance of the machine by reducing moving mass.
The various drawbacks of currently available machine designs are addressed by the present invention.